Bartlomiej Wiendlocha

Resonant levels in bulk thermoelectric semiconductors

Distortions of the electronic density of states (DOS) are a potent mechanism to increase the thermopower of thermoelectric semiconductors, thereby increasing their power factor. We review band-structure engineering approaches that have been used to achieve this, resonant impurity levels, dilute Kondo effects, and hybridization effects in strongly correlated electron systems. These can increase the thermoelectric power of metals and semiconductors through two mechanisms: (1) the added density of states increases the thermopower in a nearly temperature-independent way; (2) resonant scattering results in a strong electron energy filtering effect that increases the thermopower at cryogenic temperatures where the electron–phonon interactions are weaker. Electronic structure calculation results for Tl:PbTe and Ti:PbTe are contrasted and identify the origin of the thermopower enhancement in Tl:PbTe. This leads to a discussion of the conditions for DOS distortions to produce thermopower enhancements and illustrates the existence of an optimal degree of delocalization of the impurity states. The experimentally observed resonant levels in several III–V, II–VI, IV–VI and V2-VI3 compound semiconductor systems are reviewed.

P-type doping of elemental bismuth with indium, gallium and tin: a novel doping mechanism in solids

A new doping mechanism is described, whereby a doping impurity does not simply transfer charge to the bands of a host semiconductor or semimetal, but rearranges the core energy levels deep in the valence band of the host. This, in turn, leads to a redistribution of all electrons in the host, and, if designed properly, changes the location of the Fermi level EF and the density of conducting charge carriers near EF. The principle is proven experimentally in elemental Bi, whereby group III elements In and Ga dope Bi p-type, in spite of the fact that all three atoms are trivalent in the solid state. Electronic structure calculations show the formation of a hyperdeep defect state (HDS) and its effect on the EF in Bi doped with In (Bi:In) and Ga (Bi:Ga). The HDS at ∼5 to 6 eV below the EF of elemental Bi hybridizes with the Bi 6p electrons, and deprives the Bi valence band of two electrons per impurity atom. This then perturbs the electron count in the solid and lowers the EF. The charge on the impurity atoms is unchanged. In principle, this doping action does not result in the appearance of ionized impurities that scatter conduction electrons and holes in conventional doping. Experimentally, Shubnikov–de Haas and Hall Effect measurements show that adding In to Bi results in an increase in the density of holes. Thermoelectric, galvanomagnetic and thermomagnetic data are given for single-crystal and polycrystalline samples. In-doping leads to an enhancement of the thermoelectric figure of merit, which suggests that the new doping mechanism also gives a route to develop better thermoelectric materials. The same mechanism is at work for Sn-doped Bi (Bi:Sn), although here ionized impurity scattering is not avoided.

Fermi surface and electron dispersion of PbTe doped with resonant Tl impurity from KKR-CPA calculations

(Received 14 July 2013; revised manuscript received 24 September 2013; published 13 November 2013)We present results of a detailed study on the electron dispersions and Fermi surface of lead telluride dopedwith 2% of thallium, which is a resonant impurity in PbTe. Using the Korringa-Kohn-Rostoker method with thecoherent potential approximation (KKR-CPA), Bloch spectral functions (BSFs), which replace the dispersionrelations in alloys, are calculated, and BSF intensity maps over the Brillouin zone (alloy Fermi surface crosssections) are presented. It is shown that, close to the valence band edge, Tl does not create an isolated impurityband, but due to its resonant character, strongly disturbs the host electronic bands, leading to the disappearanceof sharp and well-defined electronic energy bands. The consequences of this effect on the transport propertiesare discussed and a qualitative explanation for the improvement in the thermoelectric properties of PbTe:Tl issuggested.DOI: 10.1103/PhysRevB.88.205205 PACS number(s): 72

Electronic structure and thermoelectric properties of p-type Ag-doped Mg2Sn and Mg2Sn1-xSix (x = 0.05, 0.1)

An experimental and theoretical study of p-type Ag-doped Mg2Sn and Mg2Sn1-xSix (x = 0.05, 0.1) is presented. Band structure calculations show that behavior of Ag in Mg2Sn depends on the site it occupies. Based on Bloch spectral functions and density of states calculations, we show that if Ag substitutes for Sn, it is likely to form a resonant level; if it substitutes for Mg, a rigid-band-like behavior is observed. In both cases, the doped system should exhibit p-type conductivity. Experimentally, thermoelectric, thermomagnetic, and galvanomagnetic properties are investigated of p-type Mg2Sn1−xSix (x = 0, 0.05, 0.1) samples synthesized by a co-melting method in sealed crucibles. Ag effectively dopes the samples p-type, and thermoelectric power factors in excess of 20 μW cm−1K−2 are observed in optimally doped samples. From the measured Seebeck coefficient, Nernst coefficient, and mobility, we find that the combination of acoustic phonon scattering, optical phonon scattering and defect scattering results in...

Electronic structure, magnetism, and spin fluctuations in the superconducting weak ferromagnetY4Co3

Results of the first principles study on the electronic structure and magnetism of the superconducting weak ferromagnet Y

Lithium as an Interstitial Donor in Bismuthand Bismuth–Antimony Alloys

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Eu2+-Eu3+ valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies

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Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α-As2Te3–xSex (0 ≤ x ≤ 1.5)

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Superconductivity of Mo3Sb7 from first principles

, are presented. Using the full potential Korringa-Kohn-Rostoker (FP-KKR) method, densities of states, dispersion curves and magnetic moments were calculated for quasi-ordered structural model of the compound in the framework of the local density approximation. Spin-polarized KKR calculations confirm that weak ferromagnetic properties of Y

Combining alloy scattering of phonons and resonant electronic levels to reach a high thermoelectric figure of merit in PbTeSe and PbTeS alloys

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